When did the first case of COVID-19 appear?

At the end of December 2019, the first case of COVID-19, the illness caused by the SARS-CoV-2 virus, was detected in the city of Wuhan in the province of Hubei. The virus rapidly spread throughout China and resulted in a pandemic. Thanks to retrospective studies and analyses, the first transmission cluster was discovered in a fish market at the end of December 2019, but isolated cases were noted on the 17^th of November and the 1^st of December. American researchers at the Universities of California and Arizona estimate today that the virus may have first appeared between mid-October and mid-November 2019. In order to determine this date, the researchers used a phylogenetic method known as a “molecular clock” that dates the common ancestor of sequenced viral genomes. Using these analyses in conjunction with epidemiological simulations, they estimated that “patient zero” could have contracted the virus at the beginning of November 2019. Although this conclusion is open to discussion, it is clear that the virus could not have appeared before October. The scientists were able to reconstruct the number of probable cases of SARS-CoV-2 infections before the virus was discovered. According to them, on the 17^th of November there could have been an average of 4 infected people, which would have become 9 by the 1^st of December. In their different simulations, the scientists showed that two thirds of simulated scenarios of zoonotic transmission to humans end with the virus becoming extinct. This underlies the difficulty of predicting the course of emerging epidemics. The first case of COVID-19 therefore probably dates from between mid-October and mid-November 2019, but the virus was only identified at the end of that year, when it was already well established in the province of Hubei. This delay underlines the difficulty of monitoring the appearance of new pathogens with high transmissibility and moderate mortality rates.